Waste gases that are likely to generate dust when treated by combustion include those which contain hazardous and combustible gases, e.g., silane (SiH4) and disilane (Si2H6) discharged from semiconductor manufacturing systems or liquid crystal panel manufacturing systems. Further, waste gases containing scarcely decomposable, global warming gases (PFCs) are also likely to generate dust when treated by combustion. Such waste gases cannot be emitted into the atmosphere as they are because they would have adverse effects upon the human body and change the global environment. Therefore, the common practice is to introduce such waste gases into a pretreatment system where the waste gas is made harmless by oxidation through combustion. For this treatment, a method is widely employed in which flames are formed in a furnace by using an auxiliary burning gas and the waste gas is burned in the flames.
In such a combustion type waste gas treatment system, the auxiliary burning gas uses hydrogen, city (natural) gas, LPG or the like as a fuel gas, and oxygen or air is usually used as an oxidizing agent. The greater part of the running cost of the system is the cost for consumption of the fuel gas and the oxidizing agent. Accordingly, how much hazardous waste gas is decomposed at high efficiency with a minimum amount of auxiliary burning gas is a measure for evaluating the performance of this type of system.
A general arrangement of a combustor used in a conventional waste gas treatment system of the above-described combustion type is shown in FIGS. 27 and 28. The illustrated combustor has a burner part 101 and a combustion reaction part (combustion chamber) 102 for oxidatively decomposing waste gas under heating at a stage subsequent to the burner part 101. The burner part 101 has a waste gas nozzle 103 opened in the center of the ceiling of the combustion reaction part 102 to introduce a waste gas G1 to be treated into the combustion reaction part 102. The burner part 101 further has a plurality of auxiliary burning gas nozzles 104 opened in the outer periphery of the waste gas nozzle 103 to introduce an auxiliary burning gas G2 into the combustion reaction part 102. A combustion gas outlet 105 is integrally connected to the lower end of the combustion reaction part 102. Thus, the waste gas G1 is passed through the center of flames annularly formed by the auxiliary burning gas G2 blown off from the auxiliary burning gas nozzles 104. While passing through the center of the flames, the waste gas G1 is mixed with the flames and thus burned. Combustion gas resulting from the burning of the waste gas Gi is discharged to the outside from the combustion gas outlet 105.
In general, the combustion reaction part 102 is defined and formed by an inner wall surface 106a of a cylindrical furnace body 106 made of a metal, e.g., a stainless steel. According to need, a thermal insulator for heat insulation is installed on the outer peripheral surface of the furnace body 106. Alternatively, a water-cooling structure is employed.
Meanwhile, the mainstream method of decomposition-treating gases containing fluorocarbons, which are considered to be causes of global warming, is heat decomposition in a high-temperature environment or -decomposition in a plasma for the time being. To use these techniques, decomposition treatment of fluorocarbon-containing gases is carried out under application of an enormous amount of energy for heating and plasma generation in decomposition treatment equipment having a complicated control mechanism for controlling a heating device, e.g., a heater, a plasma generator, a safety device, etc.
However, in the conventional example as shown in FIGS. 27 and 28, the combustion reaction part 102 is formed by the metallic furnace body 106, and the furnace body 106 is exposed to a high-temperature atmosphere of 1300° C. or more when combustion flames are formed (during operation). Therefore, the furnace body 106 wears out rapidly and cannot withstand long-term operation. In particular, when a halogen-containing gas is decomposition-treated by this system, the furnace body is etched or corroded under high temperature by a halogen gas (HCl, HF, etc.) produced after the treatment reaction and hence wears out rapidly.
When the furnace body 106 wears out in a short period of time as stated above, it is necessary to replace the furnace body 106 frequently. This causes the cost of equipment to increase. Further, when the metallic furnace body wears out, there is a danger that wear may develop in the surrounding structures (the thermal insulator, the water-cooled vessel, etc.). Therefore, it is necessary to inspect the furnace body for the degree of wear by disassembling it frequently. This causes the availability for use of the equipment to be reduced markedly and gives rise to an increase in the running cost.
Further, because the inner wall surface of the metallic furnace body 106 is heated to a high temperature by combustion flames in the combustion reaction part 102, the formation of thermal NOx is undesirably promoted by the catalytic effect of the metal. For example, this type of waste gas combustion equipment in semiconductor industry facilities is generally designed on the assumption that it will be installed in a clean room. Therefore, the equipment needs to be made compact in size. However, if a large amount of NOx is produced, it becomes necessary to separately provide a special-purpose treatment mechanism for treating the NOx. Consequently, the equipment cannot be made compact in size.
Further, in a combustor that forms combustion flames as stated above, flames are formed at the lower end of the burner part 101, resulting in a rise in temperature in the vicinity of the opening portion of the burner part 101, which is made of a stainless steel or the like. Therefore, there is a danger that the auxiliary burning gas G2 supplied to the burner part 101 may ignite and explode.
Further, when gases such as SiH4 used in semiconductor device manufacturing processes, particularly CVD processes or the like, are made harmless by a heat decomposition type waste gas treatment system, dust, e.g. SiO2, is generated. Such dust flows, together with waste gas, and adheres to the inner wall surfaces of piping and so forth, causing the exhaust pressure loss to increase. As methods of preventing the adhesion of dust to the inner wall surfaces of piping and so forth, the following methods have heretofore been available: a method wherein dust is blown off with a cleaning gas; a method wherein dust is scraped off with an intermittent manual scraper; and a method wherein a cleaning gas is always supplied through a porous inner wall to prevent adhesion of dust.
With the blow-off method using a cleaning gas, a fixed nozzle is provided over the circumferential area of piping to blow off a cleaning gas constantly or intermittently to remove dust. This method involves the problem that if the position of the nozzle is away from where dust may adhere, the dust removal effect is lessened. If a large amount of cleaning gas is supplied to maintain the dust removal effect, the cost of cleaning gas increases. In addition, because a large amount of gas flows, it is necessary to increase the diameter of piping in order to minimize the pressure loss.
With the method using an intermittent manual scraper, scraping is performed after an amount of accumulated dust has grown large. Therefore, the method requires a tank for storing large lumps of scraped dust.
With the prevention of adhesion of dust by constantly supplying a cleaning gas through a porous inner wall, it is necessary to supply a large amount of cleaning gas in order to maintain the flow velocity of cleaning gas through the inner wall throughout the piping so as to prevent adhesion of dust. Accordingly, it is necessary to increase the diameter of the piping in order to minimize the pressure loss due to the flow of a large amount of gas.
Further, the cost of cleaning gas is increasing, and it is necessary to increase the size of equipment such as a duct for exhausting the gas discharged from the pretreatment system in a building to the outside of the building.
The present invention was made in view of the above-described circumstances, and an object of the present invention is to provide a waste gas treatment system designed so that the wear of an inner wall constituting a combustion reaction part exposed to a high temperature is minimized to increase the working life, reduce the cost of equipment and improve the work availability, and the formation of NOx can be suppressed.
Another object of the present invention is to provide a waste gas treatment system designed to suppress a rise in temperature due to flames in the vicinity of the opening of a combustion burner so as to be free from the danger of explosion of an auxiliary burning gas or the like.
A further object of the present invention is to provide a waste gas treatment system capable of reliably removing dust from the inner wall surface of piping and requiring a minimum amount of cleaning gas when it is injected.